Hydraulic systems are used in a wide variety of machines, such as those used in construction, forestry and manufacturing. Over the years, a wide variety of sophisticated control systems and processes have been developed in an attempt to optimize the versatility, efficiency and overall operation of such hydraulic systems. Despite significant advances, a number of technical challenges continue to plague engineers when it comes to hydraulic system operation and control. One such challenge relates to a phenomenon known as “drift” in hydraulic actuation systems, particularly where the system is used to position and maintain a load at a specified position.
Drift may be understood as the tendency for an actuator, and hence a load positioned therewith, to unintentionally move from a commanded position over time due to leakage from or within the hydraulic system. Operators of certain machines such as loader machines, telehandlers, and similar devices will frequently be called upon to suspend a load at a desired height above the ground. Exemplary operations include accurately positioning a work implement, holding a load such as another machine at a lifted position for maintenance, or placing a load upon a structure. In many hydraulic systems, a certain degree of fluid leakage over time is well known to occur from one portion of the hydraulic system to another such as a fluid tank, or out of the hydraulic system all together. This sort of fluid leakage often results from internal component tolerances, typically necessary to allow freedom of movement. A certain amount of leakage may develop over time as the hydraulics are broken in, even in systems designed to exacting specifications.
A number of strategies addressing hydraulic actuator drift have developed over the years. One such approach utilizes a dedicated anti-drift valve. In one common anti-drift valve strategy, a pilot operated poppet valve is used to hydraulically lock a particular actuator such that fluid leakage is eliminated or at least reduced when the system is used to maintain a load at a specified position. While anti-drift valves have been relatively successful, they tend to have certain disadvantages, including slower cycle times and wasted energy due to pressure drop in the hydraulic system from valve actuation which must be compensated for by increasing hydraulic pressure and/or flow via the system pump. Anti-drift valves also tend to add cost and complexity to the overall system.
Other approaches have focused on tightening clearances of certain components within the hydraulic system to minimize fluid leakage. These strategies have also proven relatively successful, however, they are also accompanied by a number of disadvantages. For instance, tightening clearances on a main valve within a hydraulic system may result in valve sticking, where a valve component expands in response to temperature changes relatively faster than other components in the system. Using relatively tight clearances also tends to require relatively complex manufacturing processes, such as hard grinding, and matching a particular component such as a main valve to a particular hydraulic system. In other words, in systems machined to tight tolerances, it may be more difficult or impossible to random-fit the parts. There is thus a need for an improved means of compensating for leakage induced actuator drift in hydraulic systems.
A known approach to non-leakage induced perturbation of an actuator in a hydraulic system is known from United States Patent Application Publication No. 2005/0011190 A1 to Bitter (“Bitter”). Bitter provides a suspension system for the boom of a loading vehicle, having a hydraulic cylinder for raising and lowering the boom, which is controlled according to an active boom suspension process. Bitter utilizes a control valve to selectively route hydraulic oil to and from a chamber of the hydraulic cylinder. A position sensor is used to sense a position of the hydraulic piston rod, and sense when the rod is disturbed due to an inertial shock on the boom suspension system. A pressure-limiting unit separate from the main valve to the actuator is used to control fluid flow to and from the head-end chamber of the actuator such that the piston rod may be returned to an initial position following its shock-induced displacement. While Bitter may have applications in certain systems, the design is not well suited to remedying slower, undesired displacements of hydraulic cylinders, such as that induced by fluid leakage, and may not perform optimally, or at all, with certain hydraulic systems such as those having a pump with a margin control. Moreover, Bitter's use of a special pressure-limiting unit to achieve its goals increases cost, complexity and manufacturing practicability of the system.
The present disclosure is directed to one or more of the problems or shortcomings set forth above.